When uploading survey data, users can choose a threshold that suits their accuracy needs. Selecting the appropriate thresholds will help ensure the outputs generated align with the user’s expectations.
What is Ground Control Point (GCP) error?
Data processing
After you submit your survey, it enters our processing pipeline, where a complex feature-matching process occurs. Here, an algorithm detects similar points in overlapping photos and creates a sparse point cloud. A computer will georeference the sparse point cloud by marking ground control points (GCPs) in the source imagery, which adjusts the model. A geospatial specialist will intervene if the computer doesn’t mark any points, and those points will be manually marked.
After the point cloud has been georeferenced, a root mean square error (RMSE) calculation is performed to get the distance between the GCP's original input position and the estimated position of the same GCP after the adjustment is made. This distance is the GCP error.
Strict, Moderate, Lenient, or Skip all QA
The user’s choice—Strict, Moderate, Lenient, or Skip all QA—will determine what RMSE value a survey will be flagged and placed on hold. The higher the allowable GCP error, the more likely the survey will flow through our processing pipeline without delays.
The RMSE thresholds are:
| Option | RMSE Threshold |
| Strict | 5 cm (0.16 ft) |
| Moderate | 10 cm (0.33 ft) |
| Lenient | 20 cm (0.66 ft) |
| Skip all QA | No data review. Users can review GCP errors in the Processing Report. |
A geospatial specialist will perform additional photogrammetry troubleshooting if any GCP has an RMSE above the set threshold. If the error is not resolved, a geospatial specialist will place the survey on hold and contact the user who uploaded it with an explanation for:
- Why this is occurring, and
- Alternative options for processing
What about comparing to past surveys?
The final step of the Propeller processing pipeline is to compare the submitted survey with past overlapping surveys.
Strict
When the second box is checked and Strict is selected, if more than 50% of the survey area has changed by more than 0.10 m (0.33 ft), the survey is flagged and requires the intervention of a geospatial specialist. The geospatial specialist will release the survey if the changes are due to legitimate site activity. If the changes are not due to site activity and are observed in areas that appear not to have changed, then the geospatial specialist will place the survey on hold and reach out to the user who uploaded the data.
Moderate
Similarly, when the second box is checked, and Moderate is selected, a similar process occurs, but it will only occur if more than 60% of the survey area has changed by more than 0.20m (0.66 ft).
Lenient
When the second box is checked and Lenient is selected, the survey will be flagged if more than 60% of the survey area has changed by 0.40m (1.31 ft).
Skip all QA
The survey will not be compared to the previous survey. The user can make comparisons using a Volume Compare measurement.
Please note that this compare-to-past-surveys process is not mandatory and can be unchecked by the user. This means we will not compare the current survey with previous ones, which will result in a quicker turnaround time.
Slider Behavior
The Slider will default to Lenient if the data provided in the uploader fails any initial quality checks. These checks include the following:
For these initial quality checks, if Lenient is recommended and the user opts to change the slider to Moderate or Strict, it is likely the survey will be placed on hold for investigation by our geospatial specialists, resulting in a longer turnaround time.
Lenient QA required
If the data fails quality checks and warns you that you are not flying a recommended drone and camera, this will force the slider to Lenient and not allow you to change it.
Skip all QA required
If the data fails quality checks and warns you of the use of standard accuracy and with no GCPs, this will force the slider to Skip all QA and not allow you to change it.
More about skipping all QA
This option has the fastest turnaround time as it will send the survey through Propeller’s automated processing workflow. A computer will automatically mark aeroPoints and GCPs, and the survey will not be held for review by a GIS specialist for any reason.
Some GCPs may be excluded if they are not easily recognizable by a computer. We find the best results are achieved with AeroPoints or other checkerboard-style GCPs when placed on uniformly colored ground. However, the computer will still mark other styles of GCP markers. Please see our best practices for GCP markers in our article on How to Use Ground Control.
Use the Processing Report and Volume Compare measurements to assess the quality of the survey and ensure that it meets your organization’s survey requirements.
I still can't do it!
We wrote these articles to equip you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.
If you're stuck, you can connect with our support team by clicking the support button on the top right corner of your user portal.
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